The field of the invention is industrial control systems including programmable controllers, and more particularly, local area networks for linking such controllers and associated peripheral devices together.
Local area networks are comprised of two or more nodes or stations which are linked together by a communications medium. The communications medium may take a number of forms, including coaxial cable, fiber optics or twisted wire pairs. The topology of the links between stations may also take a number of forms, including star, multidrop or ring configuration.
Regardless of the medium used or the topology of the network, a control scheme is required to provide an orderly transfer of information from one station to another on the network. One such system for the orderly transfer of information between stations is the token passing system. With a token passing system, a master token in the form of a distinctive bit pattern is passed between stations on the network. While a station has the master token, it can transmit messages to other stations or command other stations to transmit messages.
In order to permit additional stations to join the network, a second "solicit" token may be passed between stations. Typically, each station holding both the master token and the solicit token may solicit other stations to enter the network. Conversely, a station holding the master token may withdraw from the network by sending a withdrawal message to its predecessor station An example of such a token passing system adapted for use in an industrial environment is given in U.S. Pat. No. 4,667,323 entitled "Industrialized Token Passing Network" issued May 19, 1987, assigned to the same assignee as the present invention and hereby incorporated by reference.
Token passing systems are desirable for industrial control applications because token rotation time and throughput can be accurately estimated provided the number and type of stations in the ring is known. U.S. Pat. No. 4,747,100, entitled: "Token Ring Network Utilizing Active Node Table" and issued May 24, 1988 discloses a token ring system where each station compiles an active node table listing the other active stations and permitting estimation of token rotation time and throughput as described above. The use of an active node table also permits simplified network maintenance, e.g. adding and dropping stations, and permits recovery from multiple station failures without reinitializing the network.
The above described token ring systems require that each station share the burden of network maintenance, i.e. adding or dropping stations from the network. If an active node table is used, each station must also have adequate memory and processing capability to support the active node table.